Proteobacteriaadjacent

Proteobacteriaadjacent is an informal designation used in microbiology to refer to bacterial lineages that are consistently found in proximity to Proteobacteria in phylogenetic analyses but are not themselves included within the phylum Proteobacteria. The term is not a formal taxonomic category and is not recognized by major authorities such as the NCBI Taxonomy database or the List of Prokaryotic names with Standing in Nomenclature. It is used mainly as a heuristic in discussions of bacterial evolution and tree topology.

In practice, Proteobacteriaadjacent describes clades that appear as sister groups or on short branches near Proteobacteria across various datasets and methods. Because it is informal, there is no fixed composition; lineages labeled as such can vary with gene selection (16S rRNA versus whole genomes), sampling density, and analytical approach. Factors underlying these near-Proteobacteria placements may include rapid early diversification, incomplete lineage sorting, or historical horizontal gene transfer.

As a concept, Proteobacteriaadjacent is employed to frame questions about the early evolution of Proteobacteria and the relationships among deep-branching bacterial lineages. It functions as a provisional placeholder rather than a definitive clade, inviting future taxonomic refinement as more data become available. Its usefulness relies on recognizing the provisional nature of phylogenetic inferences in bacteria.

See also: Proteobacteria, bacterial phylogeny, phylogenomics.